The invention relates to a method of draining water from low-consistency water-fiber suspensions, in which method the water-fiber suspension is led to a filtering surface comprising holes, through which holes water, short fiber material and possible particle-like solids can flow.
In paper-making processes, there are situations in which a fiber-containing water suspension is processed, whose consistency is low, i.e., the quantity of fiber stock is small, the suspension also contains a great deal of short fibers and possibly also particle-like solids derived from filling agents. Concentrating this kind of water-fiber suspension is quite difficult and, in practice, it is not possible to achieve efficiently with present thickeners or filters. This type of water-fiber suspension is obtained for instance by thermo-mechanical processes, whereby the freeness level of the obtained stock is approximately 70-80 ml and its consistency is low, typically approximately 1%. Further, the same situation occurs in processing process water recycled in paper-making processes where when dewatering the web during web formation, short fibers in particular and particle-like solids used in paper-making are left in the water.
It is an object of the present invention to provide a method by which short fibers and/or filling agents can more efficiently be filtered from the water-fiber suspension, thus producing a cleaner filtrate and circulating water for the process. The method of the invention is characterized in that first the rougher fiber material is separated from said water-fiber suspension, the separated rougher fiber material is led to a filtering surface to form a separate filtering layer, and the water-fiber suspension is fed through the filtering layer, whereby at least a part of the short fiber material and possible particle-like solids remains in the filtering layer formed by the rougher fibers.
In accordance with the invention, the rougher fiber material is first separated from the water suspension and fed to a thickener as a filtering layer and the suspension to be thickened is fed to the thickener or filter in such a manner that it has to flow through the filtering layer thus formed, whereby the fines and the particle-like solids remain in the filtering layer without substantially passing through the filtering surface. This type of rougher fiber material preferably has a freeness value of 200-400 ml, in which case it forms a sufficiently dense layer but still has a good water permeability.
In a preferred embodiment of the invention, two filtering layers are formed of the rougher fiber material, and the suspension is fed between them.
The invention provides the advantage that with a simple supplementary apparatus, it is possible to form of one and the same water-fiber suspension being processed a filtering layer which aids thickening by retaining fine fibers and possible particle-like solids very efficiently, making it thus possible to obtain from the thickener a filtrate whose solids content is even below 30 mg/liter. This way, sufficiently clean filtrate water, i.e., containing only a small amount of fiber material and particle-like solids, is obtained and can be used at various stages of the paper-making process.